1. Field of the Invention
The invention relates to a rotary drum vacuum filter. Particularly but not exclusively, the invention relates to the special application of such a vacuum filter for the removal by filtration of paraffin components from a mixture of mineral oil and its solvent; the invention is not restricted to this application.
2. Description of the Prior Art
In this special application, wax-like or paraffinic components are removed from for instance lubricant by temporarily diluting the mixture with a solvent, such as methyl-ethyl ketone or dimethyl ketone, in which the mineral oil dissolves and the wax components do not dissolve. On subsequent cooling of the mixture to a temperature between -10.degree. C. and -30.degree. C. the paraffin will crystallize out and can then be filtered off. For this purpose there is used a rotary drum, the wall of which is divided into sectors. Pipes connect the sectors to apertures in a rotary valve member which is opposed to a fixed valve member. The valve members cooperate, when the drum rotates, to control the supply of vacuum and pressurized gas via the pipes to the filtration sectors, in accordance with a predetermined programme. The vacuum causes suction filtering while the sector is immersed, and the pressurized gas loosens and dislodges the cake formed on the filter during filtering.
Usually the filter cake on the filter is washed even further with a solvent and is dried. At the end of each revolution of the drum, the cake is removed from the filter by first applying the pressurized gas under the cake and then moving the filter past a scraper. Typically, the drum wall is covered with a grid, over which a filter cloth is stretched.
Often two sets of pipes are used for each sector, which set being connected to a separate aperture in the rotary valve plate. One such set of pipes is connected into the drum wall near the rotationally leading edge of the sector, while the pipes of the other set emerge near the rotationally trailing part of the sector. The "front" or leading pipes provide that the sector is in an open connection with the vacuum source and with the atmosphere outside the drum as soon as the filter surface of the sector emerges from the liquid, while the "back" or trailing pipes provide that the sector remains up to the very last moment in open connection with the atmosphere when the drum plunges back again into the liquid. Also with this arrangement, liquid can collect in each sector and be drained away through whichever pipe is lower of the two.
A short period of time must elapse between the end of suction through the pipes until blowing commences, during which time the pipes can purge. Otherwise, liquid left in the pipes, which may still contain some oil, might be blown back into the filter cake.
In an attempt to increase production per vacuum filter, these filters tend to be driven faster and to acquire a larger drum length. This results in more liquid having to run out along longer lengths of pipe. An inevitable consequence, which has hitherto invariably been encountered, has been that the transition period from suction to blowing had to extend over an ever-increasing angle of rotation of the drum. The production loss on account of this increase in the angle of rotation from 3.degree. to 17.degree. required for purging, in its turn implies a considerable restriction of the production capacity of the filter.
U.S. Pat. No. 3,220,554 and U.S. Pat. No. Re. 22646 show rotary drum filters as described above, with two groups of pipes leading from the drum surface to a rotary valve member having an axial face opposed to an axial face of the fixed valve member. The apertures in the rotary valve member corresponding to the two groups are at different radii. British Pat. No. 715,251 also shows two groups of pipes, but these lead to opposite ends of the drum.